Pyrethrum flowers, their insecticidal pyrethrins component and the other pyrethroid compounds have significant insecticidal activity. Indeed, pyrethrins and pyrethrins-containing plant extracts have been used for such purposes for over a century.
The pyrethroid compounds fall into one of two classes insofar as photostability is concerned. Most of the pyrethroid compounds are decomposed by light, particularly, by ultraviolet light (290 to 320 nm). A few of the pyrethroids are quite stable to light. Those pyrethroids which are very stable to light provide a possible problem in that their use may lead to ingestion of the stable pyrethroids by animals other than insects with resulting potential damage to those animals. The photolabile pyrethroids, on the other hand, are only occasionally used outdoors since they decompose quite readily when exposed to sunlight. It would be advantageous if a pyrethroid composition could be formulated which was sufficiently light stable so as to be usable outdoors and yet which would decompose after a selected time of use outdoors so as to prevent any environmental damage, toxicity problems with higher animals, and the like.
Attempts have been made in the past to stabilize some of the photolabile pyrethroids. These attempts have generally consisted of adding antioxidants and/or light absorbers which act as uv screens in the 290 to 320 nm range (see, for example, Miskus, R. P. and Andrews, T. L., J. Ag. Food Chem. 20, 313-315 (1972); and Pieper, G. R. and Rappaport, N. L., J. Agric. Food Chem. 30, 407-408 (1982)). Another compound which has been used to photostabilize pyrethroids is p-amino azobenzene. This compound has been found to be an effective stabilizer as disclosed in U.S. Pat. No. 2,772,198, issued Nov. 27, 1956. However, many azoaromatic compounds and/or their degradation products are carcinogenic and thus their use is not appropriate since a stabilizer must be toxicologically safe as well as effective. See, for example, "Casarett and Doull's Toxicology: The Basic Science of Poisons", Second Edition, MacMillan Publishing Co., Inc., New York, 1980, as to the carcinogenic properties of azoaromatic compounds.
A better understanding of the pyrethroids and their photochemistry may be obtained by reading a review of the use of pyrethrins and pyrethroid insecticides by J. E. Casida which appears in Environmental Health Perspectives, 34, 189-202 (1980) and a review of the photochemical reactions of the pyrethroids by L. O. Ruzo which appears in Pesticide Biochemistry, Vol. 2, edited by D. H. Hutson and T. R. Roberts, John Wiley & Sons, Ltd., 1982.
While attempts to stabilize photolabile pyrethroids have met with some success, such success has only been sufficient to allow limited outdoor use of normally photolabile pyrethroid insecticides.